Fiber optic telecommunications card with energy level monitoring

ABSTRACT

A transceiver card for a telecommunications box for transmitting data over a first optical fiber and receiving data over a second optical fiber. The card has transmitter for transmitting data over the first optical fiber, the transmitter having a laser and a modulator, a fiber output optically connected to the laser for connecting the first optical fiber to the card, a fiber input for connecting the second optical fiber to the card, a receiver optically connected to the fiber input for receiving data from the second optical fiber, and an OTDR optically connected between the transmitter and the fiber output or between the receiver and the fiber input. An energy level detector is also provided between the receiver and the fiber input.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/188,643, filed Jul. 3, 2002, and claims the benefits of U.S.Provisional Patent Application 60/303,932, filed Jul. 9, 2001, bothapplications are incorporated by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to telecommunications and moreparticularly to transmitters and receivers for fiber optic networks.

2. Background Information

In current fiber optic networks, an electronic data stream is fed to anoptical fiber multiplexor, which is also called “a box” in the industry.Each multiplexor runs on a specific transmission standard, for example,SONET. A laser and an amplitude modulation circuit for the lasertypically are located on a card, which fits into the box. The laseramplitude modulator typically pulses or alters the laser output tocreate an amplitude-modulated optical signal representative of theelectronic data stream. The laser amplitude modulator and laser thusdefine a transmitter for transmitting the optical signal over an opticalfiber. A receiver for the amplitude-modulated optical signals of theoptical data typically includes a photodiode to convert the opticalsignals back into the electronic data stream. Both the transmitter andthe receiver typically are located on the backplane of a single card,which is replaceable should a component fail.

The card typically also contains a connector for receiving at least oneoptical fiber, for example a duplex SC connector. The connectorsnormally are located on a faceplate of the card, the faceplate beingperpendicular to the backplane.

The reading of the amplitude-modulated optical data signals using thephotodiode on the card is straightforward: the optical signals eitherproduce an electric output at the photodiode or they do not. As aresult, an output electronic data stream of zeros and ones is generated.

The electronics for the amplitude modulation of the laser and for thereceiving of the optical data on the card thus is relatively simple. Allthat is required is a pulsing circuit for pulsing the laser as a directfunction of the input data and a photodiode for the receiver.

Existing amplitude modulated systems have the disadvantage that thefiber can be easily tapped and are not secure.

U.S. Pat. No. 5,455,698 purports to disclose a secure fiber opticcommunications system based on the principles of a Sagnacinterferometer. A data transmitter is a phase modulator for modulatingcounter-propagating light beams sent by a receiver round a loop. Thereceiver includes a light source, a beamsplitter for splitting lightfrom the light source into counter-propagating light beams and forreceiving the phase-modulated light beams, and an output detector. U.S.Pat. No. 5,223,967 describes a similar Sagnac-interferometer-basedsystem operating over a single optical fiber.

The Sagnac-interferometer-based systems described in these patents havethe disadvantage that they require the light to travel over a loop,whether back and forth in a single fiber or over a long length loopedfiber. As a result, either the link budget for the single fiber must bedoubled, reducing the data carrying capacity for a single fiber, or elsea looped fiber with significant and expensive extra length of at leasttwice that of a single fiber must be laid between the transmitter andthe receiver. Moreover, the receiver contains the light source, asopposed to the current installed base where the transmitter has thelight source.

The Sagnac-interferometer-based systems thus are expensive to build andoperate, and do not work particularly well with existing multiplexors orcard formats.

Several companies currently provide OTDR (optical time-domainreflectometers) which can monitor an optical fiber and determine if thepresence of and location of a break in the fiber. Such companies includeYouth Communication Co. with the MW Series Mini-OTDR and OTDR modulesfrom Tekronix Communications.

These detectors however are expensive and must be applied to the fiberaway from the box.

U.S. Pat. No. 5,777,727 discloses details of OTDR technology and ishereby incorporated by reference herein.

SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide a transceiver card forproviding secure optical data transmission over optical fiber. Anotheralternate or additional object of the present invention is to providefor replacement of existing cards with a transceiver card permittingODTR and tapping detection capabilities.

The present invention provides a transceiver card for transmitting dataover a first optical fiber and receiving data over a second opticalfiber, the card having a transmitter for transmitting data over thefirst optical fiber and a receiver for receiving data from the secondoptical fiber, and an OTDR connected optically to the second opticalfiber upstream from the receiver. Preferably, the OTDR operates at awavelength that is different than the wavelength used for datatransmission and is connected in the optical circuit via a wavelengthdivision multiplexed coupler. By operating the OTDR at a wavelengthdifferent than the wavelength used for data transmission, the OTDR maybe allowed to continuously operate without disruption of the datatraffic.

Preferably, an energy level detector is also provided on the card. Theenergy level detector is preferably optically connected to the secondfiber between the OTDR and the receiver.

The OTDR preferably is monitored by an embedded processor within thebox. The processor analyzes the OTDR output data to determine if thecharacteristics of the optical fiber system have changed relative to areference characteristic. If the optical fiber system has changedrelative to the reference OTDR characteristic, then a degradation of theoptical fiber system, which may indicate a fiber breach or a fiber tap,can be logged along the position of the degradation source relative tothe location of the OTDR. The indication of degradation of the fibersystem may provide an alarm signal, for example an electronic signalsent to a network operations center to indicate the measured degradationof the fiber system along with the approximate location of thedegradation point, a light on the outside of the box or a sound-emittingalarm. Alternatively, the OTDR output may be connected to a displaymodel via an electrical connection on the card for display to a humanoperator, or transferred by the transceiver to be analyzed at thenetwork operations center.

The transceiver card of the present invention preferably has fiber spanlength ranges for operation that are compatible with standard opticalmultiplexor operation, for example, two to twenty kilometers or ten toone hundred kilometers. The OTDR and energy level detector must have ameasurement dynamic range that ensures proper operation over the spanlength limits of the transceiver card. By specifying fiber span lengthranges for the OTDR and energy detector enhanced transceiver, the costof implementation of the OTDR and energy level detector can be optimizedwith span length thus providing an optimized cost of implementationbenefit to the customer.

The present invention thus permits a card-based transmission systemincorporating an energy level detector for optical tap detection, whichcan provide for more secure data transmission than existingamplitude-based cards along with breach localization services from theOTDR. Because of advances in semiconductor and optical componentpackaging, the OTDR and energy level detector parts along with theoptical transmitter and receiver components can fit all on one cardcompatible with most existing box dimensions.

The transmitter light source preferably is a laser, for example asemiconductor laser operating at a 1550 nm, or other, wavelength.

Preferably, the energy level detector provided on the card for measuringlight energy in a fiber is connected electronically to an alarm, so thatwhen a drop or increase in the energy level is detected, which mayindicate a tap, the card may provide an alarm signal, for example anelectronic signal sent to a network operations center to indicate a dropor increase in the optical energy level, a light on the outside of thebox or a sound-emitting alarm. Depending upon the optical transmissionmethod implemented, a successful tap may be placed by adding light tothe system through the tapping device. Implementations of the singlefiber Sagnac transmission method described in U.S. Pat. No. 5,223,967may be susceptible to such a tapping method unless an energy leveldetector that monitors for an increase or decrease in the optical signallevel is included as part of the design.

The card includes an optical fiber interface for at least one fiber, andpreferably for two fibers. The interface may be a duplex SC connector,for example.

The card preferably is a replacement part for an existing opticalmultiplexor transceiver card.

The present invention also provides a method for providing a continuallyoperating or, preferably, a commanded operation OTDR within an existingbox including the steps of:

removing an existing transceiver card; and

replacing the transceiver card with the card of the present invention.

The present invention also provides a method for manufacturing anoptical transceiver card for transmitting data over at least one datatransmitting optical fiber, the card having a transmitter and areceiver, the method comprising the steps of:

placing a transmitter on a printed circuit board,

placing a receiver on a printed circuit board; and

placing an OTDR on the printed circuit board.

Preferably, an energy level detector is also placed on the printedcircuit board, and a light is connected to a faceplate connected to theprinted circuit board, the light indicated a change in energy at thedetector or degradation of the optical fiber system.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described below byreference to the following drawings, in which:

FIG. 1 shows schematically a card of the present invention located in anexisting telecommunications box, such as a mutliplexor; and

FIG. 2 shows a block diagram of the transceiver of the presentinvention.

FIG. 3 shows a description of an analog energy level detector of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 shows an existing telecommunications box 2, for example amultiplexor, refitted with a card 1 of the present invention. Box 2 hasan electronic data input 3 and output 4, which connect to a motherboard5 of the box 2. Motherboard 5 includes a bus 6 for connecting existingamplitude-based cards to the motherboard 5, and connects the input 3 andoutput 4, through for example, data conversion circuitry, to the bus 6.The type of bus 6 is dependent upon the box manufacturer, and differenttypes of boxes, motherboards and buses are well known in the art. Card 1of the present invention includes electrical connections 8 to fit intobus 6.

Card 1 also includes a faceplate 9 and a backplane 7, which preferablyis a printed circuit board, Faceplate 9 may be perpendicular tobackplane 7 and be flush with a front side of box 2.

Faceplate 9 may have a fiber connector 109, such as a duplex SCconnector, for connecting to an output fiber 110 and an input fiber 111.Alternately, a single fiber for inputting and outputting signals couldbe provided.

FIG. 2 shows the card 1 of the present invention in more detail. Atransmitter 10 transmits signals over optical fiber 110. Transmitter 10includes a single laser 12, for example a semiconductor laser emitting anarrow band of light at approximately 1550 nm, or at other wavelengths.Light emitted from laser 12 passes through a modulator 16, for examplean amplitude or phase modulator, directly next to or part of the samepackage as laser 12. The light may be depolarized by a depolarizer 14.An electronic controller 18, preferably manufactured directed on theprinted circuit board of backplane 7 (FIG. 1), controls modulator 16 andmay provide power to laser 12 Input data 19 is fed to the controller 18,which then controls modulator 16 to modulate the light from laser 12 asa function of the input data 19.

The transceiver of the present invention preferably operates in aphase-modulated mode, though conventional amplitude-modulatedtransmitters and receivers, including those using return-to-zero typesignals, for example, may also be used. The phase-modulated signals havethe advantage that breach detection by the energy level detector workmore effectively, since the amplitude of the optical signal is constantand thus a drop in the optical signal level is more easily detected.

Optical signals are received at connector 109 from fiber 111.

Receiver 11 includes two coupler/splitters 31 and 131, each functioningas a splitter. Splitter 131 is preferably a wavelength divisionmultiplexed coupler/splitter to allow the OTDR 132 to operate at oneoptical wavelength, for example 1670 nm, while the transmitted datastream 19 and received data stream 34 are carried on a differentwavelength, for example 1550 nm. This functionality allows the OTDR 132on transceiver card 1 to be commanded to continuously operate withoutinterruption or corruption of the received data stream 34. Splitter 131splits off the wavelength of light applicable to the OTDR into fiber133, which has an input to OTDR 132. Splitter 31 then splits off aportion of the remaining other light, directing part of the opticalenergy to an energy level or tap detector 33 and passes the residuallight to an optical receiver 32. Optical receiver 32 converts theoptical signal from optical to electronic form to recover the electronicdata stream 34 as appropriate for the optical modulation techniqueemployed.

OTDR 132 has a control circuit 134 and a bus 135 which allows the deviceto be controlled by a processor. The OTDR thus can monitor the fiber 111and provide information through bus 135 to a processor for determiningthe location of a breach or tap.

Detector 33 monitors the light energy in the fiber 111 via the lightenergy coupled to the detector by splitter 31. If the amplitude dropsduring monitoring, which may indicate a tap, the detector 33 provides analert and can, for example, send and electronic signal to the processorvia bus 135 to indicate a drop or increase in the optical energy level,sound an alarm or alert network maintenance personnel, for examplethrough an LED 133 or by sending an alarm message using transmitter 10.Another LED 134 can provide an indication of proper signal reception. Anenergy level detector control circuit 233 controls the alarm thresholdand energy detection and provides output indications from the energydetection circuit to a processor via bus 135 which may be shared withthe OTDR control circuit 134.

FIG. 3 shows the energy level detector 33 of the present invention inmore detail. The energy level detector 33 described by FIG. 3 representsa preferred analog implementation, with other implementation circuitspossible, for bounding the optical energy within an acceptable rangewith thresholds which may be programmable.

A photodetector or other optical to electrical conversion device 153measures the optical signal coupled to its input by coupler/splitter 31.The output of photodetector 153 is an electrical voltage whose levelcorrelates to the optical power at the input to the photodetector 153based upon the photodetector 153 transfer optical to electricalconversion transfer function. Depending upon the electrical bandwidth ofphotodetector 153 and the optical signal format present at the input tophotodetector 153, the electrical signal may be filtered by a low passfilter 154 to provide an average voltage level which represents theaverage optical power measured by photodetector 153. After filtering thesignal, the electrical signal may be conditioned and scaled by either alogarithmic or linear amplifier 155. Scaling the data may be necessaryto ensure that energy level detection can be made without performancedegradation over the span length range required for the circuit. Thechoice of scaling type is chosen primarily based upon the optical toelectrical conversion transfer function of the photodetector and therange of expected optical power levels at the photodetector 153 inputbased upon span length ranges. Generally, the transfer function ofsemiconductor photodetection devices is exponential with respect tooptical to electrical conversion. For such components, the cascade of anexponential photodetector with a logarithmic amplifier offers theadvantage of providing a net linear transfer function from optical powerat the input to the photodetector to voltage at the logarithmicamplifier. Thus, a digitally programmable detection threshold can bedeveloped which offers the same resolution per bit regardless of thespan length of the device.

The electrical signal, after being scaled by the linear or logarithmicamplifier 155, is compared to reference voltages by one or morecomparators. As shown in FIG. 3, comparator 156 will transition from alow to high output when the voltage output from the logarithmic orlinear amplifier 155 exceeds the reference voltage established by thedigital to analog (D to A) converter 158. Conversely, comparator 157will transition from a low to high output when the voltage output fromthe logarithmic or linear amplifier 155 falls below the referencevoltage established by the digital to analog converter 159. The outputof OR gate 160 will transition froth low to high when either the outputof comparator 156 or comparator 157 transitions from low to high. Forthe example of FIG. 3, an alarm state is said to exist when the outputof OR gate 160 is high. To indicate an alarm state, the OR gate outputmay trigger an audible alarm via a siren 162, a visual alarm via a lightor light emitting diode (LED) 161 or may indicate an alarm state to theprocessor via the energy level detector interface 233 and processor bus135. The reference voltages established by D to A converters 158 and 159may be programmable through a digital processor or state machine via adigital bus 135 and an energy level detector interface circuit 233. Oneor more thresholds 163 and 164 may be established to provide referencelevels for comparison to determine one or more alarm states. Thus, thecircuit of FIG. 3 may be configured to monitor in real time the opticalpower at the receiver 11 for excess light or too little light toindicate a potential optical tap, tamper or other degradation of theoptical signal.

A digital circuit equivalent to FIG. 3 may be developed. Analog todigital conversion of the logarithmic or linear amplifier 155 outputfollowed by comparison of the digital result to digital thresholdseither via software or digital hardware would indicate optical energylevels within limits or not. Additional filtering via averaging ofdigital conversions via a moving average or other digital filteringtechnique could replace or supplement filtering provided by analogfilter 154. A digital implementation also offers information regardingan estimate of the measured optical signal power, both peak and average,by monitoring the analog to digital output via a computer interface.

The component sizes and types will depend upon the type of transmissiondesired. For example OC-3 cards may vary in size and certain componenttypes from OC-192 cards.

While the cards may be placed in new boxes, the present invention alsopermits for the removal of existing optical transmission cards to beeasily replaced by the enhanced security cards. The fibers aredisconnected, the box 2 is simply opened and theamplitude-modulated-based card is removed. The card 1 is inserted intothe bus 6 and the fibers are connected.

The card 1 of the present invention may thus provide existing boxes withcontinual breach localization and detection secure transmission modecapability.

Moreover, a separate OTDR which is space-consuming need not be provided.While the energy level detector must be at the receiver side, the OTDRalso could be located on the transmitter side.

1-17. (canceled)
 18. A telecommunications apparatus, comprising: anoptical receiver affixed to a printed circuit board and configured toreceive an optical data signal from an optical fiber of an optical fibertelecommunications system; an energy level detector circuit configuredto: monitor an energy level of the optical data signal; and generate analarm based on the energy level and one or more energy level thresholds.19. The telecommunications apparatus of claim 18, wherein the alarmincludes a visible alarm indicated by a light emitting diode (LED). 20.The telecommunications apparatus of claim 18, wherein the alarm includesan audible alarm indicated by a siren.
 21. The telecommunicationsapparatus of claim 18, wherein the alarm includes an electronic signalprovided to a processor via a processor bus.
 22. The telecommunicationsapparatus of claim 18, wherein the energy level detector circuit isoptically coupled to the optical fiber upstream from the opticalreceiver.
 23. The telecommunications apparatus of claim 22, wherein theone or more energy level thresholds include a first energy levelthreshold for detecting a particular drop in the energy level and asecond energy level threshold for detecting a particular increase in theenergy level.
 24. The telecommunications apparatus of claim 22, whereinthe optical data signal comprises a phase-modulated optical data signaland wherein the optical receiver is configured to obtain data from thephase-modulated optical data signal.
 25. The telecommunicationsapparatus of claim 22, wherein the energy level detector circuitincludes: a photodetector to generate a photodetector voltage indicativeof an energy level of the optical data signal; one or more comparatorscorresponding to the one or more energy level thresholds, wherein eachof the one or more comparators: includes a first input coupled to anoutput voltage indicative of the photodetector voltage; includes asecond input coupled to a corresponding reference voltage; and generatesa comparator signal indicative of a comparison between the correspondingreference voltage and the output voltage.
 26. The telecommunicationsapparatus of claim 25, further comprising: one or more digital-to-analogconverters corresponding to the one or more comparators, wherein eachdigital-to-analog converter receives a reference input and generates areference voltage based on the reference input.
 27. Thetelecommunication apparatus of claim 26, further comprising: logicconfigured to receive an output from each of the one or more comparatorsand further configured to generate an alarm signal based on the outputfrom each of the one or more comparators
 28. The telecommunicationsapparatus of claim 26, further comprising: a control interfaceconfigured to communicate with a processor to enable programmablereference inputs.
 29. The telecommunications apparatus of claim 25,further comprising: a low pass filter, coupled to an output of thephotodetector, to produce an average value signal indicative of anaverage value of the energy level of the optical data signal.
 30. Thetelecommunications apparatus of claim 29, further comprising: anamplifier configured to amplify the average value signal to produce anamplified signal, wherein the amplified signal comprises the outputvoltage received by each of the one or more comparators.
 31. Thetelecommunications apparatus of claim 22, further comprising: an opticaltransmitter affixed to the printed circuit board and configured to:receive a data stream; and generate an outgoing optical data signalindicative of the data stream; and transmit the outgoing optical datasignal via an optical fiber optically coupled to the opticaltransmitter.
 32. The telecommunications apparatus of claim 31, whereinthe optical transmitter includes: a laser; a modulator; controlelectronics to provide control signals, in accordance with the datastream, to the laser and modulator to modulate the laser in accordancewith data stream; and a depolarizer configured to receive an output ofthe modulator.
 33. The telecommunications apparatus of claim 22, furthercomprising: an optical time-domain reflectometer (OTDR) affixed to theprinted circuit board and optically connected to the optical fiberupstream from the energy level detector circuit; and a processorconfigured to: analyze OTDR information from the OTDR to monitor one ormore characteristics of the optical fiber telecommunications system; andresponsive to detecting a change in at least one of the one or morecharacteristics of the optical fiber telecommunications system,determine a position of a degradation source relative to a location ofthe OTDR.
 34. The telecommunications apparatus of claim 33, furthercomprising: a wavelength division multiplexed (WDM) splitter configuredto split an OTDR signal, having a first wavelength, from the opticaldata signal, wherein the energy level detector circuit is opticallycoupled to the optical fiber between the WDM splitter and the opticalreceiver.
 35. A telecommunications method, comprising: receiving, by anoptical receiver affixed to a telecommunications card and opticallyconnected to an optical fiber, an optical data signal from the opticalfiber; monitoring, by an energy level detector affixed to thetelecommunications card and optically coupled to the optical fiberupstream from the optical receiver, an energy level of the optical datasignal; providing a first reference signal corresponding to a firstenergy level threshold and a second reference signal corresponding to asecond energy level threshold; and generating an alarm, based on theenergy level of the optical data signal, the first reference signal andthe second reference signal, responsive to detecting either: the energylevel exceeding the first energy level threshold; or the energy levelbelow the second energy level threshold.
 36. The telecommunicationsmethod of claim 35, wherein the alarm is selected from: a visible alarmindicated by a light emitting diode (LED); an audible alarm indicated bya siren; and an electronic signal provided to a processor via aprocessor bus.
 37. The telecommunications apparatus of claim 35, whereinmonitoring the energy level of the optical data signal includes:generating, by a photodetector, a photodetector voltage indicative ofthe energy level of the optical data signal; providing an output voltageindicative of the photodetector voltage, a first reference voltagecorresponding to the first reference signal, and a second referencevoltage corresponding to the second reference signal; comparing, by afirst comparator, the first reference voltage and the output voltage;comparing, by a second comparator, the second reference voltage and theoutput voltage; and asserting an alarm state signal responsive todetecting either: the output voltage exceeding the first referencevoltage; or the output voltage below the second reference voltage. 38.The telecommunications method of claim 37, wherein providing the outputvoltage indicative of the photodetector voltage comprises: low passfiltering the photodetector voltage to generate an average value signalindicative of an average value of the photodetector voltage; andamplifying the average value signal to produce the output voltage. 39.The telecommunications method of claim 38, wherein anoptical-to-electrical transfer function of the photodetector isexponential and wherein amplifying the average value signal comprisesamplifying the average value signal with a logarithmic amplifier andwherein a net transfer function of the photodetector and amplifier islinear.
 40. The telecommunications method of claim 37, furthercomprising: programming the first reference voltage by providing a firstprogrammable input to a first digital-to-analog converter configured togenerate the first reference voltage and providing a second programmableinput to a second digital-to-analog reference voltage by providing asecond programmable input to a second digital-to-analog converterconfigured to generate the second reference voltage.